Most modern vehicles include a vehicle radio that requires an antenna system to receive amplitude modulation (AM) and frequency modulation (FM) broadcasts from various radio stations. Present-day vehicle antenna systems may include a mast antenna that extends from a vehicle fender, vehicle roof, or some applicable location on the vehicle. Although mast antennas provide acceptable AM and FM reception, it has been recognized by vehicle manufacturers that the performance of a mast antenna cannot be significantly increased, and therefore, improvements obtained in other areas of in-vehicle entertainment systems will not include reception capabilities of the mast antenna. Consequently, vehicle manufacturers have sought other types of antenna designs to keep pace with consumer demands for increased vehicle stereo and radio capabilities.
Improvements in vehicle antenna systems have included the development of backlite antenna systems, where antenna elements are formed on a rear window of the vehicle in various designs. Backlite antenna systems have provided a number of other advantages over mast antenna systems, including no wind noise, reduced drag on the vehicle, elimination of corrosion of the antenna, no performance change with time, limited risk of vandalism, and reduced cost and installation.
A new concept for antenna systems has been invented to provide an antenna between the inner and outer laminated glass sheets of a vehicle windshield. U.S. Pat. No. 5,528,314, entitled "Transparent Vehicle Window Antenna" issued Jun. 18, 1996, and U.S. Pat. No. 5,739,794 entitled "Vehicle Window Antenna With Parasitic Slot Transmission Line," issued Apr. 14, 1998, disclose "Solar-Ray" antennas of this type.
FIG. 1 is a diagrammatic view of a Solar-Ray vehicle antenna 10 of the type disclosed in these patents laminated in a windshield 12 of a vehicle 16. FIG. 2 is a diagrammatic view of the windshield and solar-ray antenna 10 removed from the vehicle 16. The windshield 12 is mounted within an opening of a vehicle body 14 that is made of an electrically conductive metal, such as steel or aluminum, by known window mounting techniques. The windshield 12 includes a horizontal dark tinted region 18 formed along a top border of the windshield 12 that reduces glare for the vehicle operator. The translucent nature of the tinted region 18 can be used to reduce the visibility of the antenna 10.
The antenna 10 is provided in the windshield 12 as a conductive film applied to the inner surface of an outer glass of the windshield 12 to be contained between outer and inner glass layers of the windshield 12. The film of the antenna 10 is essentially transparent to visible light, highly reflective of infrared radiation, electrically conducting, and preferably has a sheet resistance of 3 ohms per square or less. An example of a suitable film material is described in U.S. Pat. No. 4,898,789 to Finlay, issued Feb. 6, 1990. The film described herein can include a first anti-reflective metal oxide layer, such as oxide of zinc and tin, an infrared reflection metal layer, such as silver, a primer layer containing titanium, a second metal oxide layer, a second infrared reflective metal layer, such as silver, another primer layer, a third anti-reflective metal oxide layer, and an exterior protective layer of titanium metal or titanium oxide.
The antenna 10 includes two basic elements--a horizontally elongated tuning element 20 substantially parallel to and spaced from a top edge 22 of the window 12, and an impedance matching element 24. The tuning element 20 is essentially rectangular, although its horizontal edges may follow the curvature of the window edge 22 and its corners may be rounded for a more pleasing appearance. The tuning element 20 has an effective horizontal length of an odd integer multiple of one-quarter of the wavelength to which it is tuned, and thus exhibits a zero reactive impedance at the tuned wavelength. Different tuning element configurations can be provided in different designs. In one embodiment, the tuning element 20 is tuned to a wavelength in the center of the FM frequency band (88 MHz-108 MHz), such as three meters, and thus has an effective horizontal length of about 0.75 meters. The physical length of the element 20 at resonance is actually somewhat shorter than one-quarter of the center frequency of the FM band to provide coupling to the vehicle body 14. The length by which the element 20 is shorter will vary with the specific vehicle application. In one particular vehicle, the tuning element 20 has been found to work well with a horizontal length of 60 cm and a vertical width of 50 mm. The element 20 is ideally spaced below the window edge 22 by a distance which provides maximum FM gain. However, this distance may be compromised to gain other advantages for a particular vehicle design. The antenna 10 provides AM reception through capacitive coupling with the vehicle body 14.
The impedance element 24 includes a main body portion 28 which covers substantially all or most of the windshield 10 below the tinted region 18 to provide FM impedance matching. In the '794 patent, the impedance element can be a ribbon in various configurations to form a parasitic slot transmission line for FM impedance matching purposes. The main portion 28 has a peripheral edge 32 with a horizontal upper portion 34 spaced at least 25 mm below the lower edge of the element 20, so as to minimize transmission coupling effects therebetween. The upper portion 34 is connected to the element 20 by a narrow vertical portion 36 to provide an electrical current flow. The upper portion 34 of the peripheral edge 32 is preferably within the tinted region 18 of the windshield 12 along its entire length from one side to the other side of the windshield 12, so that the tinted region 18 overlaps the main portion 28 of the element 24. The remaining portion of the peripheral edge 32 is spaced a certain distance from the edge of the vehicle body 14 so as to provide, in combination therewith, a planar slot transmission line that is parasitically coupled to the element 20. In one embodiment, the distance between the edge of the vehicle body 14 and the main portion 28 is preferably within the 10-25 mm range. The length of the slot is substantially an integer multiple of one-half of the wavelength to which the tuning element 20 is tuned, so that each end of the slot transmission line, at the junctions of the upper portion 34 and the remaining portion of the peripheral edge 32, appears as an electrical open circuit.
The impedance element 24 is used to adjust the real component of the antenna's impedance to match the characteristic impedance, typically 125 ohms, of the coaxial cable used to feed the antenna 10. This is accomplished by the predetermined width between the remaining portion of the peripheral edge 32 and the adjacent portion of the edge of the window 12. For appearance purposes, and to maximize the infrared reflecting efficiency of the windshield 12, an opaque painted band 40 may be provided around the sides and bottom of the windshield 12 to substantially or completely cover the area outward from the remainder portion of the peripheral edge 32 to the outer edge of the windshield 12. This band can be broken into dots of decreasing size toward the inner boundary for a fade-out effect, as known in the industry. If such a band is provided in combination with the tinted region, substantially the entire viewing area of the windshield 10 can be uniformly provided with the infrared reflecting film of the antenna 10.
The impedance element 24 also provides an added benefit at AM wavelengths. At these longer wavelengths, the antenna 10 is not a resonant antenna, but is substantially a capacitive antenna. The large area of the element 24 provides a substantial boost in gain for the antenna 10, as compared with similar planar and other antennas in the prior art. In fact, the boost in AM gain is so great that some of it can be sacrificed, if desired, in fine tuning the antenna performance for further improvements in FM gain, directional response, or other characteristics while still yielding good AM performance.
In order to connect the antenna 10 to a radio or other communications system, a connection arrangement is necessary for an external coaxial cable. In this embodiment, the antenna 10 is extended into a narrow strip 38 (about 25 mm wide), upward from the center of the element 20, and almost to the upper edge of the windshield 12. An inner conductor 42 of a coaxial cable 44 is electrically connected to the narrow strip 38 to feed the tuning element 20. In one embodiment, the inner conductor 42 is electrically connected to a patch 46 formed on an inside surface of the inner layer of the windshield 12. An outer conductor 48 of the coaxial cable 44 is connected to the vehicle body 14 at a convenient point close to where the inner conductor 42 is coupled to the feed point. Any suitable feed connection can be provided between the tuning element 20 and the center conductor 48 of the coaxial cable 44 within the skilled of the art.
Vehicle antenna reception and performance can be increased by providing compensation or correction for broadcast reception when the primary feed system provided by the narrow strip 38 is positioned at a null location, and the antenna 10 is prevented from receiving suitable signal strength. For example, buildings, mountains, etc., may cause reception problems when the vehicle is positioned at a certain location. The reflection of the FM signals from the various bodies in the reception area causes the signals to interact with each other creating null locations from destructive interference. This type of antenna null is caused by the spatial location of the feed system. Other types of nulls are caused by signal configuration as a result of signal pattern and polarization.
To address this problem, it would be desirable to increase antenna performance by providing feed diversity for the nulls caused by signal interference. This feed diversity should include space diversity, pattern diversity and polarization diversity. It is known to provide multiple vehicle antennas on the vehicle with the idea that if one of the antennas is positioned at a signal null location, the other antenna may be in a location to receive suitable signal strength. The vehicle antenna orientation relative to the signal determines how well the reception is achieved relative to signal strength. However, providing multiple antennas positioned at different locations of the vehicle significantly increases the cost of the vehicle antenna system.
What is needed is an FM diversity feed in combination with a primary antenna feed for a single vehicle antenna that meets desirable antenna cost and complexity. It is therefore an object of the present invention to provide such a secondary antenna system.